1. Field of the Invention
The present invention relates to a method of preparing a thin film of an oxide superconductor containing Tl, and more particularly, it relates to a method of preparing a Tl--Bi--Sr--Ca--Cu--O, Tl--Ba--Ca--Cu--O or Tl--Pb--Ba--Ca--Cu--O oxide superconducting thin film.
2. Description of the Background Art
An oxide superconducting thin film is generally prepared by physical vapor deposition such as RF sputtering, ion beam sputtering, laser ablation or electron beam vapor deposition. In such vapor deposition, an oxide crystal of Al.sub.2 O.sub.3, MgO, SrTiO.sub.3, LaAlO.sub.3 or LaGaO.sub.3 is employed as a substrate. An oxide thin film having a prescribed composition is vapor-deposited on the substrate.
Generally in the Tl-based superconducting thin films, whichever method is employed, the as-grown oxide thin film is in an amorphous phase exhibiting no superconductivity immediately after the vapor deposition. After the vapor deposition, therefore, it is necessary to heat treat the amorphous film at a temperature of about 600.degree. to 950.degree. C., in order to crystallize to superconducting phase. It is effective to heat treat the amorphous thin film under the Tl vapor for the crystallization of Tl--Bi--Sr--Ca--Cu--O, Tl--Ba--Ca--Cu--O or Tl--Pb--Ba--Ca--Cu--O systems. Since there are many superconducting phases having different number of Tl--O or Cu--O layers, however, it is necessary to form the single superconducting phase.
In the Tl--Bi--Sr--Ca--Cu--O system, a superconducting phase having the highest critical temperature is expressed in a composition formula of (Tl, Bi)Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x (1223 phase), and its critical temperature is about 120 K. In addition, there exists a superconducting phase having a lower critical temperature, which can be expressed in a composition formula of (Tl, Bi)Sr.sub.2 CaCu.sub.2 O.sub.x (1212 phase). When a heat treatment is performed in accordance with the conventional method to obtain a thin film of the 1223 phase in this system, however, the 1212 phase is preferentially formed in general. According to the conventional method, no 1223 phase is formed unless the heat treatment is performed at a high temperature of about 900.degree. C. However, the heat treatment under such a high temperature is followed by aggregation of the structure and formation of impurities, leading to extreme reduction in volume rate of the as-formed 1223 phase. Further, the high-temperature heat treatment also leads to decomposition, resulting in deposition of a granular Ca--Cu--O phase. Once such a granular phase is formed, this phase remains so stable that formation of the 1223 phase is inhibited. In addition, the high-temperature heat treatment leads to dissociation of Tl.
In the Tl--Ba--Ca--Cu--O system, on the other hand, superconducting phase having the highest critical temperature is expressed in a composition formula of Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x (2223 phase), and its critical temperature is about 125 K. In addition, there exists a number of superconducting phases having lower critical temperatures, which can be expressed in composition formulas of Tl.sub.2 Ba.sub.2 CaCu.sub.2 O.sub.x (2212 phase), TlBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x (1223 phase) and TlBa.sub.2 Ca.sub.3 Cu.sub.4 O.sub.x (1234 phase). When a heat treatment is performed in accordance with the conventional method to form the 2223 phase in this system, however, the 2212 phase is preferentially formed in general and formation the 2223 phase is suppressed. In general, therefore, it is difficult to form the 2223 phase having the highest critical temperature at a high volume rate. When the neat treatment is performed at a high temperature to form the 2223 phase, it leads to the aforementioned problems of separation of a Ca--Cu--O phase and dissociation of Tl.
In the Tl--Pb--Ba--Ca--Cu--O system, further, a superconducting phase having the highest critical temperature is expressed in a composition formula of (Tl,Pb)Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x (1223 phase), and its critical temperature is about 120 K. In addition, there exists a superconducting phase having a lower critical temperature, which can be expressed in a composition formula of (Tl, Pb)Sr.sub.2 CaCu.sub.2 O.sub.x (1212 phase). When a heat treatment is performed in accordance with the conventional method to form the 1223 phase in this system, however, the 1212 phase is preferentially formed in general. Also in this system, therefore, it is difficult to form the 1223 phase at a high volume rate by the conventional method. When the heat treatment is performed under a high temperature, further, it also leads to the aforementioned problems of separation of a Ca--Cu--O phase and dissociation of Tl.